| The research area, with an area of ca.100km2, lies in the southern Jinhu sag of the Subei basin. The main exploration targets are Funing Fm. and Dainan Fm. In this paper, on the basis of fully digesting achievements and knows of predecessors, and of the modern theory of hydrocarbon accumulation, and of integrated application of a large number of analysis data, we have analyzed reservoir forming conditions, established reservoir formation mode, summarized reservoir forming regularity, and analyzed the cause of the poor shows in Batian, after which, favorable prospects are predicted.In this paper, applying the theory and method of geochemistry, we have systematically analyzed and evaluated hydrocarbon migration and accumulation in the peripheral regions of the Longgang sag. The evaluation of the three sets of source rocks in the Funing Fm. shows that, as the main source rocks in this area, source rocks of the E1f2member and the E1f4member generally have high abundance of organic matter, with type II as the primary organic type, and are ranked as good-better source rocks. However, hydrocarbon source rocks of the E1f3member are organic-lean, whose hydrocarbon generation potential is limited, contributing little to hydrocarbon generation. The burial depth of the Longgang sub-sag is larger, and its source rock thermal evolution degree is higher, reaching mature stage at present and having strong ability to generate hydrocarbons. The source rock thermal evolution in the Chajian sub-sag, the Dongyang sub-sag and the Batian region is at low mature stage.Physical properties of crude oil are obviously different in different blocks of the research area. Both density and viscosity of crude oil are lower in the peripheral regions of Longgang sub-sag, whereas they are relatively higher in the Chajian sub-sag and the Dongyang sub-sag. Characteristics of crude oil have differences in different layers of the research area, indicating that they have different sources. Sterane distributions of crude oil in the E1f2member and the E1f3member, sourced from source rocks of the E1f4member, are dominated by "anti-L" pattern. Crude oils have different maturities in different layers of the research area. Among them, crude oils in the peripheral regions of Longgang sub-sag have higher maturity, belonging to hydrocarbon products generated at mature stage of source rock; they should be from source rocks at deep sag. Crude oils have lower maturity in the peripheral regions of Chajian sub-sag and Dongyang sub-sag, and in the Batian region, belonging to hydrocarbon products generated at low thermal evolution stage, in accord with their early mature source rocks; they should be sourced from local source rocks.Sterane distributions and maturity of crude oil are obviously different in the E1f2member of TianX96well and Tian33-2Well, indicating that they have different sources, that mature crude oils from Longgang sub-sag haven’t migrated across the fracture to the TianX96well region, and that the fracture is in a closed state between TianX96well and Tian33-2Well, inhibiting mature crude oil from migrating and accumulating in the west. Sterane distributions of crude oil are basically the same at different depths of the E1f2member of TianX96well; crude oils have a little higher maturity in the deep layers than in the lower layers, being consistent with its source rock thermal evolution degree. Characteristics of crude oils in the E1f2member of TianX96well are obviously different from that of in the Tian33-2well; they should belong to hydrocarbons self-generated and self-accumulated in the source rock of TianX96well. Reservoirs in the BaTian region contain oil under the fluorescent microscope, and the oil has lower maturity, belonging to hydrocarbon products generated at low evolution stage, which are consistent with its source rock thermal evolution degree and should belong to hydrocarbons self-generated and self-accumulated in the source rock of the BaTian region.It is the primary cause of the poor shows. |
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